1. Field of the Invention
The present invention relates to structural steel plates having markedly improved resistance to stress corrosion cracking caused by nitrate as often encountered by the shell plates of hot stoves, boilers and high temperature heating furnaces which generate NO.sub.x.
Generally in hot stoves and high temperature heating furnaces, the combustion gases containing nitrogen oxides, such as NO, NO.sub.2 and N.sub.2 O.sub.4, condensate on to the inner shell surfaces forming nitrate when they are cooled below their dew points. It is well known in the field that when the shell, conventionally made of structural low-alloy steels containing 1.0% or less Cr, Ni and V, is contacted with this nitrate solution, it is very often susceptible to stress corrosion cracking.
This cracking phenomenon has been called "nitrate stress corrosion cracking (nitrate SCC)", and causes a more and more crucial problem to be solved in modern hot stoves and heating furnaces where the amount of NO.sub.x is ever increasing as the treating temperature is raised.
For clear understanding of this phenomenon, the description about a hot stove is made in FIG. 1.
When a hot stove is subjected to nitrate SCC in actual operations as shown in FIG. 1, it has been found that the cracking mostly occurs near a weld zone which includes the weld for jigs during construction, and highly stressed portions.
The cracks initiate at inner surface of a hot stove shell as illustrated in FIG. 2 and propagates toward through-thickness direction.
FIG. 3 shows the cross-sectional view of nitrate SCC.
In nitrate SCC of structural steels, various factors such as (1) the presence of nitrate and a temperature; (2) the external stress plus residual stress and (3) segregation of certain elements and carbide formation along grain boundaries have been considered to be entangled.
As described before, nitrate SCC initiates at hard spots near the weld zone and propagates along grain boundaries into matrix toward through-thickness direction as shown in FIG. 3.
In particular, the nitrate SCC depends on the localized corrosion due to the segregation of such elements as C, N and P, and carbides formed along grain boundaries, which become a preferential dissolution path for SCC.
The present inventors have found that, in addition to the above factors, a fourth one, the ability of the steel to be repassivated upon its chemical breakdown of the passivity in the presence of a very small amount of chloride ion contained in the combustion gases.
Based on the above findings, the present inventors have succeeded in developing a new structural steel plate by controlling the repassivation process in addition to the reduction of segregation along grain boundaries for preventing the cracking of the shell.
The stable passive film formation of steels is closely related to the steel composition, particularly contents of Cr and Mo.
Also it has been found that co-presence of Cr and Mo further improves high temperature strength and the toughness of the thick plate to a great extent. By these reasons, Cr and Mo are essential elements.
On the other hand, the control of the (C+N) content is a very useful means in reducing the segregation and carbides formation along with grain boundaries.
At a weld zone, however, additional remedy has to be made particularly when Cr is added to the steel since Cr depletion may occur at grain boundaries in the heat affected zone (HAZ) due to the formation of Cr carbides upon welding, which leads to a preferential dissolution path for nitrate SCC.
The formation of Cr depletion can be completely avoided by the addition of strong carbide formers such as Nb.
The addition of Nb and the control of the ratio of Nb/(C+N), the degree of carbide stabilization are indirectly effective for the enhancement of the repassivation ability at the weld zone of Cr-containing steels through the elimination of Cr depletion, which becomes a preferential dissolution path, by the formation of Nb carbide and nitride at higher temperature than Cr.
The importance of Cr depletion in preventing SCC of low alloyed structural steel plates is firstly noticed by the present inventors.
Meanwhile, since the shell of a hot stove or a heating furnace is usually a large welded structure, it is needless to say that the steel must satisfy the requirements of mechanical properties, weldability (including hardenability, resistance to mechanical cracking and joint properties), workability (gas cutting and bending) and economical requirements in addition to the above resistance to nitrate SCC both in the base plate and the weld zone.
Among all, the weldability and the gas cutting property are most important for commercial grades of steels in practical service: From the point of weldability, the content of carbon is severely restricted, while, from the point of gas cutting property, the chromium content is also severely restricted.